The label printers of the kind mentioned are frequently used for printing labels on the reel, where one of the objectives of manufacturers of such label printers is to achieve the most efficient use of the ribbon used in printing in order to optimize the costs of the label-printing operation.
This entails equipping the label printer with means for controlling the advance of the ribbon, and making the resultant information available to a control device that controls the movements of the printing head and the take-up or advance rate of the ribbon.
In some label printers, a pull-roller unit is used to advance the ribbon in order to achieve accurate winding. Such accurate tangential ribbon pulling is achieved by controlling the pull-roller unit, though this requires a mechanism that is subject to wear and tear and requires maintenance owing to friction.
Other label printers arrange ribbon advance through simpler mechanical means, by having the take-up spool driven directly, where the rotation of said spool is controlled. This technique poses problems of inaccuracy in calculating the advance of the ribbon, since the linear travel of the ribbon depends not just on the rotation of the take-up spool but also on the radius of said take-up spool, and that radius becomes larger as the amount of ribbon wound around the take-up spool increases.
Also known, through patent JP-A-61-199972, is the technique of detecting ribbon movement all along the feed path of the ribbon to offset changes in the radius of the take-up spool, though that patent describes a label printer with a stationary printing head rather than a movable one.
A description is given in patent EP 76907576 or ES 2 132 902 of a method for calibrating a ribbon-winding mechanism for a printing device in which the ribbon is advanced by means of a stepper motor acting on the ribbon take-up spool. In that patent, the calibration is arranged by using a means of detection that measures ribbon advance when the take-up spool is turned, where the means of detection is connected to a control device. The means of detection comprises a free-spinning roller or ‘idler’ that is kept in contact with the ribbon and has a magnetic, optical or other such system for sending the necessary information about the advance or linear travel of the ribbon to a suitable receiver.
The use of this type of idler can give rise to problems if the ribbon slips on the roller—which can happen when the printing device is used in a packaging area, especially when the product to be packaged is powdered and features a low friction coefficient. In this case, the ribbon can slip or slide over the idler, making it turn irregularly or not at all, giving a completely false measurement of ribbon advance.
As mentioned above, this aim of attaining an accurate measurement of the ribbon's advance has a decisive influence on both the efficient use of the ribbon and on successful labelling or printing, since if ribbon advance is not perfectly controlled, a section of the ribbon that has already been used may be used again for printing another label, thus giving defective or incomplete printing.
In general, such label printers perform the thermal transfer of the ribbon's printing medium onto the surface to be labelled at a rate of one label per cycle, and making the ribbon advance between each printing action.
In cases where the printing of more than one label per cycle is required, this technique usually entails an excessively inefficient use of the printing ribbon, thus making the labelling process more costly.